1. Field of the Invention
The present invention relates to an exposure apparatus and a device fabrication method.
2. Description of the Related Art
A projection exposure apparatus has conventionally been employed to fabricate a micropatterned semiconductor device such as a semiconductor memory or logic circuit by using photolithography. The projection exposure apparatus projects and transfers a circuit pattern formed on a reticle (mask) onto a substrate such as a wafer via a projection optical system.
Along with the recent advance in micropatterning of semiconductor devices, the projection exposure apparatus is being required to further improve the resolving power (a minimum feature size that the exposure apparatus can transfer) than ever. To meet this demand, the wavelength of the exposure light is shortening, and the development of a high-NA projection optical system is in progress (the numerical aperture (NA) of the projection optical system is increasing). For example, a projection exposure apparatus which uses an ArF excimer laser beam having a wavelength of about 193 nm as the exposure light and includes a projection optical system having an NA more than 0.8 attains a resolving power of 0.1 μm. Also, an immersion exposure apparatus has already been proposed, which increases the NA of the projection optical system by the so-called immersion method of filling at least a part of the space between the wafer and the final surface (final lens) of the projection optical system with a liquid having a refractive index higher than 1. Under the circumstances, a further improvement in the resolving power than ever is being expected.
To achieve an improvement in the resolving power of the exposure apparatus, it is indispensable to precisely evaluate the performance (for example, the imaging performance) of the projection optical system. Conventionally, the transmitted wavefront aberration has been mainly used to evaluate the performance of the projection optical system as mounted in the exposure apparatus or that of the projection optical system during assembly. Along with an improvement in the performance of the projection optical system, the influence of the birefringence of the projection optical system is becoming non-negligible. To keep up with this recent trend, it is necessary to evaluate not only the transmitted wavefront aberration but also the birefringence (the birefringence on the pupil plane) of the projection optical system.
To meet this need, Japanese Patent Laid-Open No. 2006-214856 proposes a measuring apparatus in which an interferometer for measuring the transmitted wavefront aberration in assembling the projection optical system is additionally imparted with a function of measuring the birefringence of the projection optical system. The measuring apparatus disclosed in Japanese Patent Laid-Open No. 2006-214856 mounts an imaging optical system in order to measure (evaluate) the birefringence (birefringence distribution) of the projection optical system. To accurately measure (evaluate) the birefringence of the projection optical system, it is necessary to isolate the birefringence of the imaging optical system (to be referred to as the “system error” hereinafter) mounted in the measuring apparatus, and that of the projection optical system.
The birefringence is generally represented by a Jones matrix, so matrix calculation is necessary to isolate the system error and the birefringence of the projection optical system (that is, to calibrate the measuring apparatus). For example, Japanese Patent Laid-Open No. 2006-214856 separately measures the birefringence of the forward path from the light source to the projection optical system (an optical system inserted in the optical path from the light source to the projection optical system), and that of the backward path from the projection optical system to the light detection unit (an optical system inserted in the optical path from the projection optical system to the light detection unit). Then, the Jones matrix representing the measurement value containing the system error and the birefringence of the projection optical system (that is, the measurement result obtained by the measuring apparatus) is multiplied by the inverse matrices of the Jones matrices representing the birefringences of the forward and backward paths, thereby isolating the system error and the birefringence of the projection optical system.
Unfortunately, Japanese Patent Laid-Open No. 2006-214856 discloses merely a technique used in assembling the projection optical system, and discloses no technique of measuring the birefringence of the projection optical system as mounted in the exposure apparatus, and isolating the system error of the measuring apparatus used in measuring that birefringence. In other words, this patent reference proposes no technique of accurately measuring the birefringence (birefringence distribution) of the projection optical system as mounted in the exposure apparatus.
The technique disclosed in Japanese Patent Laid-Open No. 2006-214856 inserts a mirror and prism at positions at which the incident angle of light is 5° or less in the imaging optical system mounted in the measuring apparatus, divides the optical path in the measuring apparatus into four, and measures the birefringences of these optical paths, thereby obtaining the birefringences of the forward and backward paths. Therefore, to isolate the system error and the birefringence of the projection optical system, birefringence measurement must be performed a number of times (at least four times), which requires a long measurement time.